Sapphire and silicon carbon are used as substrates for many different electronic and optical devices formed using the gallium nitride (GaN) material system. The gallium nitride material system refers to alloys of indium (In), gallium (Ga), aluminum (Al), and boron (B) with nitrogen to form a nitride. Typically, the devices are fabricated by forming layers using various compositions of indium, gallium, aluminum, boron and nitrogen to create various alloy compositions. These alloys are typically grown, or deposited, as epitaxial layers over a substrate. These epitaxial layers are typically formed using growth techniques such as metal organic chemical vapor deposition (MOCVD), also referred to as organo-metallic vapor phase epitaxy (OMVPE) or other techniques. Typical materials formed using the gallium nitride material system include indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN), aluminum indium gallium nitride (AlInGaN) and others. Typical devices formed using the gallium nitride material system include, for example, transistors and light emitting devices that emit light from about the ultraviolet (UV) portion of the electromagnetic spectrum to the blue and green visible portion of the electromagnetic spectrum. Light emitting diodes that emit light in the blue and green portion of the visible spectrum and laser devices that emit blue light have active regions comprising gallium nitride and indium gallium nitride.
To make an optoelectronic device, many material layers are grown upon one another over a substrate until the device is completed. Depending on the particular composition, each material layer has what is referred to as a “critical thickness.” The critical thickness is dependent on the substrate used, the composition of the material forming the layer and the growth conditions under which the layer is grown. One way of defining the critical thickness is the thickness at which an epitaxially grown material layer begins to develop dislocations. Dislocations are defects in the crystal lattice of the material. Dislocations form when a lattice parameter mismatch exists between adjacent material layers. The dislocations degrade the optical quality of the material. When forming these layers epitaxially, it is often desirable to grow the layers as thick as possible while preserving the crystal structure so that low-defect material having high optical quality is obtained. By growing the material layers well below their critical thickness, the lattice mismatch between the adjacent layers is compensated by elastic strain. However, this may limit the usefulness of a device.
Further, using indium gallium nitride (InGaN) grown on gallium nitride (GaN) as an example, increasing the indium content to extend the wavelength of an optical device increases the lattice mismatch between the gallium nitride and the indium gallium nitride, thus limiting the thickness of the indium gallium nitride material layer. Both the thickness and indium concentration in indium gallium nitride alloys are limited because of the lattice mismatch with the underlying gallium nitride. Unfortunately, no currently available substrate material is lattice matched to indium gallium nitride.